Thermal energy storage device

ABSTRACT

A heat-storage battery device, comprises a cover, closing components, and an inner separating surface that along with said cover, defines an inner volume that provides a continuous pathway for materials inside the inner volume of the device, and simultaneously allows for enhanced melting.

FIELD OF THE INVENTION

The invention relates to a thermal energy storing device that comprises phase-change materials.

BACKGROUND OF THE INVENTION

Phase-change materials (hereinafter also referred to as “PCMs”) are often used in the field of thermal energy, since they have the ability to absorb heat even at small temperature differences with the environment, due to their thermal properties, such as latent heat.

The absorption of heat can be used for thermal energy storage, and in the case of PCMs a relatively large amount of thermal energy can be absorbed in relation to the mass and volume of the PCMs. A device that comprises a PCM can be used as a “thermal battery”, since heat can be discharged when the use of thermal energy is required. A common way for charging a PCM battery is by exposing it to heat that originates from sun radiation, thus storing the solar energy.

One disadvantage of PCMs is that PCMs with high latent heat usually have low thermal conductivity. According to the prior art, a possible solution for increasing the thermal conductivity in devices that contain PCMs is the use of separating surfaces (fins) with high thermal conductivity between PCM layers, which provides a better conductivity within the device. Such surfaces create a separation between layers of PCMs and each volume between two surfaces acts as a separate cell of a PCM battery. Most surfaces, according to the prior art, are circular or longitudinal, but in each case the surfaces prevent a continuity of the PCM along the device.

When melting (charging) takes place, the heat transfer rate from the source of heat to the PCM usually decreases with time. This is because a layer of molten liquid between the fin and the solid PCM grows with time, creating an increasing thermal resistance.

Another disadvantage of PCM-comprising devices is that the volume of PCMs changes according to the amount of absorbed or discharged heat. When charging the materials with thermal energy the heated material expands. The expansion of materials inside a device can cause stress on different components of the device that are in contact with the expanding material, and as a result can sometimes cause mechanical failure. When discharging heat, PCMs undergo solidification and as a result the volume of the material decreases, creating air voids that redefine the shape of the material inside the device, which can result in an uneven solidification and reduced heat transfer area.

When using separate cells of PCM batteries, as suggested in the prior art, each cell must be provided with a void in which the material can expand during melting. In addition, any adjustment, such as replacing the material inside the device, has to be performed on each cell separately, which obviously complicates the use of the device and increases operation costs.

Therefore, it is an object of the present invention to provide a device that comprises heat conductive surfaces that improve the heat transfer within a PCM-based device.

It is another object of the invention to provide a device that comprises a single cell in which PCMs can be inserted, while maintaining heat-transfer improving surfaces.

Other objects and advantages of the invention will become apparent as the description proceeds.

SUMMARY OF THE INVENTION

The invention relates to a heat-storage battery device, comprising a cover, closing components, and an inner separating surface that along with the cover, defines an inner volume that provides a continuous pathway for materials inside the inner volume of the device. Such materials are usually PCMs that are suitable for heat storage. The separation surface can be shaped as a helix or as any other surface suitable to permit close-contact melting (CCM), while (1) providing a continuous inner pathway for materials that are positioned inside the device, and (2) having a large surface area comparable to that of circular or longitudinal fins. In embodiments of the invention close-contact melting is achieved using an inner separating surface, which is a helical surface coiled around an inner core, such as a pipe, which surface has an inclination that is as small as possible that the mechanical configuration permits. A quasi-horizontal surface, when possible, provides the best results for CCM.

The invention can further comprise a pipe that is located within the device, for example, the separating surface can be provided around the pipe. The inner volume of the pipe is suitable to allow a fluid (liquid or gas, including steam) to flow therein.

The closing components are adapted to seal the inner volume of the device from the environment, and the cover and the separating surface are in contact to prevent any leak of material from the sides of the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

1. FIG. 1 is a perspective view of a separating surface and a pipe, according to one embodiment of the invention;

2. FIG. 2A is a front view of the separating surface of FIG. 1, showing a vertical cross-sectional axis A-A;

3. FIG. 2B is a view of the section of FIG. 2A taken along the AA plane;

4. FIG. 3 is an exploded view of the separating surface of FIG. 1 and the other components of the device, according to one embodiment of the invention; and

5. FIG. 4 is a front view of the assembled device of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

An illustrative type of materials that are suitable for heat storage is phase-change materials, in which the density of the material changes when absorbing or discharging heat. An exemplary PCM used for heat storage is NaNO₃ because of its high volumetric heat capacity, which indicates a high ability for heat storage. The change of the volume of the materials when absorbing heat (expanding) or when discharging heat (shrinking) requires a suitable void within the device that hosts the material that can accommodate the material in all phases.

FIG. 1 is a perspective view of separating surface 101 and pipe 102, according to one embodiment of the invention. Separating surface 101, which can also be referred to as a “fin”, is shaped as a helix, thus providing a continuous volume into which PCMs can be inserted. Pipe 102 is suitable to allow a flow of materials through its inner volume, such as heated water, and it can be used for heat transfer between the PCM and the material that flows through pipe 102. Pipe 102 can be connected to other components or to a water source, for example.

The use of separating surface 101 provides a one-cell battery device wherein all of the material that is located within the device is in contact with the continuous surface, thus significantly improving heat transfer to the PCM. The shape of surface 101 provides an increased heat transfer area, which also increases the rate of heat transfer, which in turn results in faster charging (when the material is heated) and discharging (when the material releases heat during solidification). It is also possible to use convection to increase the heat transfer rate.

Melting from the outer cover or shell (i.e., melting as a result of heat transferred to the PCM at the outer surface of the device) as well as through the inner pipe 102 would lead to a situation in which the solid becomes surrounded by the molten material from all directions, and thus sinking of the solid would occur and a so-called “close-contact” melting would take place above the separating surface, which is only moderately inclined, i.e. close to horizontal, thereby increasing the rate of melting and, unlike in the prior art units, keeping the rate of melting almost constant throughout the entire process.

Due to the shape of surface 101, the continuous volume within the device allows the PCM to easily expand and shrink during different thermal processes. According to this embodiment there is a need for only one void for future expansion since there is only one “cell” that contains the PCM. During melting, all of the material concentrates at the bottom, due to gravity, so there is no separation of the material.

During melting the excessive volume of liquid created by the phase change is conveyed into the upper part of the shell, whereas during solidification the shrinkage at any point inside the unit is compensated by the liquid flowing from above due to gravity. Thus, both high pressures at melting and voids at solidification are excluded inside the unit.

FIG. 2A is a front view of separating surface 101 and pipe 102 of FIG. 1, showing a vertical cross-sectional axis A-A, and FIG. 2B is a view of the section of FIG. 2A, taken along the AA plane, both showing the pathway through which materials can flow. Surface 101 is not provided along the whole length of pipe 102 in order to leave a void for the material that is located within the device for when it expands, and because pipe 102 can be connected at its edges to other components, such as sealing component, as will be shown in FIGS. 3 and 4.

Apart from separating surface 101 and pipe 102, the device comprises other components, as shown in FIG. 3 in an exploded view, such as cover 301. Surface 101, pipe 102 and cover 301 define the inner volume of the device in which PCM can be filled. Cover 301 can be made of any material that is suitable to be in contact with the specific PCM that is used in a specific device, and insulated from outside. Moreover, as shown in FIG. 4, the outer edge of surface 101 and cover 301 may be in contact, thus causing the material to flow along the continuous formed pathway while utilizing the largest possible heat transfer area.

FIG. 3 also shows sealing components (flanges) 302 a, 302 b, 303 a, and 303 b. Components 302 a and 302 b are suitable to be connected to cover 301 by a screw mechanism, and components 303 a and 303 b are suitable to connect to component 302 a and 302 b by screws that can be positioned inside holes such as hole 304. Pipe 102 is also suitable to be connected or to be in contact with sealing components 302 a, 302 b, 303 a, and 303 b, which can be replaced with any other closing (and not necessarily sealing) components that have the ability to connect to the other components of the device and separate the inner volume of the device from the environment.

All the above description has been provided for the purpose of illustration and is not meant to limit the invention in any way. Many different shapes and sizes of the contact surfaces, pipes, connecting and sealing elements, etc. can be devised by the skilled person, and many different construction materials known to the man of the art can be employed, along with different PCMs, without exceeding the scope of the claims. 

1. A heat-storage battery device, comprising a cover, closing components, and an inner separating surface that along with said cover, defines an inner volume that provides a continuous pathway for materials inside the inner volume of the device.
 2. A device according to claim 1, wherein the inner separating surface is suitable to allow close-contact melting (CCM) of the phase-change material above it.
 3. A device according to claim 1, wherein the separating surface is shaped as a helix.
 4. A device according to claim 1, further comprising a pipe.
 5. A device according to claim 4, wherein the separating surface is provided around the pipe.
 6. A device according to claim 4, wherein the inner volume of the pipe is suitable to allow a fluid to flow therein.
 7. A device according to claim 6, wherein the fluid is liquid.
 8. A device according to claim 6, wherein the fluid is gas or steam.
 9. A device according to claim 1, wherein the closing components are adapted to seal the inner volume of the device from the environment.
 10. A device according to claim 1, wherein the cover and the separating surface are in contact. 